Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver salt photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver salt photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive (thermal) transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive (thermal) transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the heat-sensitive transfer sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the heat-sensitive transfer sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
Examples of the properties of the heat-sensitive transfer sheet and the heat-sensitive transfer image-receiving sheet necessary for such an image-forming method include: performance of spectral characteristics desired in color reproduction; easy transfer; fastness to light and heat; resistance to various chemicals; easy synthesis; and being easy to prepare the heat-sensitive transfer sheet and the heat-sensitive transfer image-receiving sheet. In this image-forming method, a dye left on the heat-sensitive transfer sheet after image-recording is not used. Accordingly, it has been desired to develop a dye showing a high transferring rate from the heat-sensitive transfer sheet to the heat-sensitive transfer image-receiving sheet at the time of heating with a thermal head.
JP-B-6-19033 (“JP-B” means examined Japanese patent publication) discloses specific pyrazolone-series dyes, and JP-A-10-305665 (“JP-A” means unexamined published Japanese patent application) discloses specific bispyrazolone methane-series dyes. However, these publications do not disclose any such dye-transferring that can enhance by employing any dye and a specific image-receiving layer in the image-forming method using the thermal transfer system, at all.